The present invention relates generally to a keyless entry system for operating one or more vehicle devices, such as a door lock device, a trunk lid opener and so forth, by means of a radio waves carrying a preset code or codes and transmitted between a controller mounted on a vehicle and a pocket-portable transmitter carried by a user. More specifically, the invention relates to an antenna for radio-wave communication between the controller and the transmitter by way of electromagnetic induction. Furthermore, the invention relates to a mounting structure for the antenna for radio-wave communication.
In the recent years, a new keyless entry system for automotive vehicles has been proposed and put on the market. This system does not require mechanical key operation or manual entry of a preset code to operate various vehicle devices, such as the vehicle door lock, the trunk lid opener and so forth. In this keyless entry system, a pocket-portable transmitter is used as a source of a preset code signal. The transmitter generates a radio-wave carrying the preset code and transmits the preset code-carrying radio-wave to a controller mounted on a vehicle. The controller receives the radio-wave and decodes the preset code in the radio wave. When the decoded code matches a preset code in the controller, a control signal is sent from the controller to the vehicle device to be operated.
To establish radio-wave communication between the transmitter and the controller, antennas are provided both for the transmitter and the controller. In the proposed system, radio-wave communication between the controller and the transmitter is generally short-range. For instance, unlocking the door lock or opening of the trunk lid are necessary only when the user is actually near the vehicle. Furthermore, it would be comfortable for the door to be unlocked when the user is standing just in front of the door or for the trunk to be opened when the user is standing just behind the vehicle. In this view, the radio-wave communication between the transmitter and the controller is usually performed within a range of less than 1 m. For such relative short-distance communication, electromagnetic induction can be conveniently used and would be desirable in view of the lower power requirements for radio-wave commuication than in other communication systems.
The electromagnetic induction between the controller and the transmitter must be reliable whenever transmission of codes between the controller and the transmitter is ordered. However, as is well known, electromagnetic induction is influenced significantly by the polarity of the magnetic fields around the controller and the transmitter. For instance, when the magnetic flux is essentially parallel to the antenna axis, the sensitivity of the antenna is significantly decreased, which often leads to failure of communication. On the other hand, the space for mounting the antenna on the vehicle is quite limited. Therefore, the size of the antenna is strictly limited. On the other hand, since the transmitter is of the size of a bank card or credit card for convenience, the size of the antenna is strictly limited.
Therefore, a compact and effective antenna for an automotive keyless entry system is required.